1. Field of the Invention
The invention relates to the conversion of an original data scan rate to a scan rate differing from the original and more particularly to the conversion from one scan rate to another by a non-integer factor.
2. Description of the Prior Art
High definition TV is a new picture transmission method which wil achieve significant improvements in picture quality over that presently provided. This enhanced picture reception is established by creating a picture of the highest quality at the studio. Transmitting a signal of such high quality over a distribution channel requires an excessive bandwidth and methods are being investigated to reduce required bandwidth without a significant degradation in received picture quality. Common to all the methods under investigation is the reduction of the total number of lines transmitted for each picture. To accomplish this without picture degradation requires a conversion from the original scan rate to a scan rate reduced for transmission. Since pixel on the horizontal scan lines are vertically aligned, scan rate conversion may be accomplished by vertically sampling the pixels, each pixel being a sample of the picture. Thus, scan rate conversion is a processing method akin to sampling rate changes in digital signal processing.
An analog signal may be digitally processed by sampling the time analog signal at a suitable rate. Once the sequence of samples is established the sample rate may be decreased by selecting every Mth sample of the original sequence, thus increasing the sampling period. In this process M is an integer and if the original sampling period is T the new sampling period is T'=MT. The original sampling produces a sequence of repetitive discrete-time Fourier transforms which are contiguous in the frequency domain if the sampling was at the Nyquist rate, i.e. sampling rate equal to twice the highest frequency in the Fourier transform of the time signal sample. Increasing the sampling period causes the discrete-time Fourier transform in the sequence to overlap and the highest frequencies of the transform to be reflected into the lower frequencies of the discrete-time transfrom (aliasing). Under these circumstances distortion may be avoided by passing the original sequence through an ideal low pass filter to cut off the offending high frequencies prior to the selection of each Mth sample of the sequence.
A sequence x(n) originally sampled with a period T can be converted to a new sequence which appears to have been sampled with a period T"=T/L by inserting L-1 zero samples between each original sample. To produce this new sequence, however, a digital low pass filter must be used to clear the zero stuffed spectrum of unwanted spectral components in the frequency range between .pi./T and L.pi./T. This zero stuffing establishes a Fourier transform for the new sequence that is a periodic continuation of the Fourier transform for the original sequence, the period being 2.pi./T rather than 2.pi./T" realized with a sampling period T". To produce a signal spectrum that appears sampled at T" it is necessary to provide a filter for removing the spectral images at 2m.pi./T, where m=1,2, . . . (L-1).
When the sampling rate is altered by a factor L/M that is not an integer the decimation process of reducing the sampling rate by an integer is applied to the interpolation process which previously increased the sampling rate. To accomplish this every Mth sample of the interpolated sequence having a sampling rate L times that of the original sequence is selected. The effects of decimation on the interpolated spectrum results in repetitions of the original spectrum at k(L/M) (.pi./T) intervals without changing the spectral shape of the shifted Fourier transforms. If L/M is less than unity spectral repetitions overlap. To eliminate this overlapping the interpolation filter is designed to bandlimit the spectrum to (L/M) (.pi./T). To provide a sampling conversion of L/M, that is not an interger, it is therefore necessary to zero stuff sampling of the original sequence, pass the resulting spectrum through an appropriate filter, and then decimate the resulting sequence. In preforming this process a new sequencee is made L times longer than the original sequence. This requires the new sequence to be stored in L memories L times faster than the original sampling rate to prevent overflow, providing a filter that is L times faster than the original filter, and selecting but M of L available scan samples. This process is wasteful both of data and processing time.